How Strict is the Max Vsupply Calculation for Steppers?

[Neale]

One possible reason for different temperatures for the two motors on your Y axis is whether or not you have "half current when stationary" selected on both. A lot of stepper drivers have this option available - it cuts motor current to half normal value if the motor is just holding position and not moving. I'm only guessing here - I would expect the DIP switches on both Y drivers to be set identically.

[routercnc]

My thoughts would be to unplug both Y motors from the drivers (power off!) and do a simple check of the resistance through the pairs of windings in both motors. They should all be about the same, around a few ohms.

If you are de-rating one of the steppers on a dual stepper axis just go easy on the feeds to avoid stalling one out before the other (assuming they are just slaved in software)

[mattnedgus]

I would expect the DIP switches on both Y drivers to be set identically.

Hi Neale, yeah all DIP switches are the same.

My thoughts would be to unplug both Y motors from the drivers (power off!) and do a simple check of the resistance through the pairs of windings in both motors. They should all be about the same, around a few ohms.

If you are de-rating one of the steppers on a dual stepper axis just go easy on the feeds to avoid stalling one out before the other (assuming they are just slaved in software)

I may check the resistances in the future but once the current setting was lowered again the difference in temperature wasn’t anywhere near as bad - maybe 5-7degreesC.

I’m not sure I understand what you mean by de-rating one of the steppers? Both are set the same to half step and a lower current (I think this might be what you mean by de-rating?) to manage the heating. I spoke to Gary at Zapp who set me straight on matching/setting to the the Peak current on the motor driver to the current rating of the motor but still remain confused, especially when I’ve read Leadshines advice is to match/set to the RMS column.

I’ve run the motors fine by setting the motor drivers Peak current value to match the specified motor current for a few years but I’d hoped to better understand why it’s done this way.

All I can figure is that I can either have higher voltages for more torque at faster speeds, but I have to set a lower current (match the motor current to the Peak value on the driver) to manage the heating OR I can set the drivers to match the RMS value to the motors rating and have a lower voltage, again to manage the heating - I ‘suppose’ then I’d get higher lower speed torque, but lower top speeds/top-end torque due to the lower (thus slower) voltage changes?

I could do with a good reference text!

[mattnedgus]

...the corresponding external case temp of the motor would likely be over 100C...

Hi Muzzer, thanks for your explanation! Sorry I'd not seen your response when I just made my last reply.

It sounds like I was maybe OK with a 90+degC case temperature for a short time then (but it didn't half heat the garage up!).

Would I be right in thinking that:

When using higher voltages (within driver spec), the only real reason to set the motor drivers Peak current to match the motors rated current is to keep the temperature down? Higher voltages are preferred to achieve higher-speeds when direct driving because current flows in and out faster, letting the coil get to a higher current/field strength/torque per step.

- e.g. I supply 68V and I set motors (rated at 4.2A RMS) to 4.3A Peak in the driver (equivalent to 3.1A RMS in the driver on the Leadshine DM856)

And so conversely:

If I wanted to set the motor drivers RMS current to match the motors rated current, I'd need to keep the voltage down below those values generated by Mariss Friemans max Voltage equation to manage the temperature? Greater current flowing in the coil would produce greater torque, but having to run at a lower voltage would mean lower top-speeds.

- e.g. I supply 50V and I set motors (rated at 4.2A RMS) to 4.0A RMS in the driver (equivalent to 5.6A Peak in the driver on the Leadshine DM856)

I've seen so many good but contradicting views on whether to use the Peak or RMS values but I don't understand the fundamental reasoning behind them and this is the best I can figure out at the moment.

[Muzzer]

I really wouldn't know the answer to your questions without knowing more about the drivers you are using. However, the peak current is what the motor can tolerate without magnetic saturation, whereas the RMS current is what causes the electrical heating , which is almost entirely due to resistive losses in the copper windings.

To complicate matters, the "better"(?) drivers reduce the driving current after a short period, once the motor has hopefully reached its target. This allows high movement torque but reduced steady state heating. And if you go further ie closed loop steppers, the control scheme is more sophisticated so that the motor current is a more direct function of the applied torque ie no more than it actually needs to be.

Leadshine are a solid outfit, so in the event of conflicting information, I'd go with what they say, not least if you are using their drivers. I expect there are application notes by Leadshine explaining not just how to set up their drives but possibly even giving some insight into what the settings do and mean. I've used their closed loop steppers in the past and was quite impressed with the level of documentation provided.